Younger age at menarche is a strong risk factor for breast cancer. For each year of decline in age at menarche, risk of breast cancer has been estimated to increase 4%. In most developed countries, age at menarche has declined, on average, 3 months per decade over the last century, whereas in developing countries, the decline has been more rapid. Postulated reasons for the younger age at menarche include improved nutrition, lower levels of physical activity and the concomitant rise in obesity. Understanding the impact of physical activity has been hampered by crude measurement, however, and studies to date have not examined rigorously the relative impact of physical activity versus weight. To examine the impact of physical activity on onset of menarche, we will use data from the Growing Up Today Study (GUTS) a longitudinal study of 16,882 children who have completed annual surveys since inception of the study in 1996 when they were 9-14 years of age. Using age at onset of menses as our outcome, we will examine the impact of physical activity, independent of the effect of body weight. We will also examine the effects of overweight and inactivity, and their effect modification, on timing of menses. Understanding the predictors of menarche will help to develop a public health approach to the issue of early pubertal development.